This invention relates generally to thin film electronic display or imager devices and more particularly to repair lines contained in devices such as liquid crystal displays or solid state imagers having a matrix of electrically conductive transmission lines for controlling the active components of the device.
Transmission lines for transmitting electrical signals to and from active components in a display or imager device are formed as integral parts of the structure of liquid crystal displays (LCDs) and solid state imagers. Such lines are also sometimes referred to as address lines. For ease of discussion, "display device" is used herein to refer to active cells of all types, including both LCDs and photosensors such as are used in imagers. These transmission lines usually form a matrix, with lines running in one direction designated as scan lines and lines running in a substantially perpendicular direction designated as data lines. The combination of the electrical signals (e.g. the voltage) on a scan line and a data line control a switching device, such as a field effect transistor (FET). The FET in turn controls the application of voltages to an active cell (pixel). In an LCD, for example, pixels contain liquid crystals disposed between transparent electrodes; the voltage applied to the electrodes governs the orientation of the liquid crystals which in turn determines the amount of light transmitted through the cell. Alternatively, the active component may be a photosensor, such as a photodiode, which generates an electrical output in response to incident optical photons. Application of the appropriate control signals to selected FETs in the array of photosensors allows the charge accumulated on corresponding photosensors from incident light to be read and directed to the processing circuitry. The resultant signals on the scan and data lines may then be used to electronically reproduce an image of the light detected by the array of photosensors.
A defect on a scan or data line can adversely affect overall performance of the thin film display device. For example, an open circuit condition can disable active components connected to the line beyond the point where the open circuit exists. A short circuit between a data and a scan line can also lead to inaccurate signals being applied to all switching transistors connected to either of the shorted data or scan lines. In either case, multiple pixels in the device can be affected, thereby significantly diminishing its resolution. A device having defective transmission lines may have to be discarded, depending upon the degradation of the resolution of the display device resulting from the inoperative pixels resulting.
Given the expense of fabricating thin film electronic display devices, it is desirable to have devices that are repairable after they have been assembled. In one common approach, a thin film display device has several auxiliary conductive lines disposed across the transmission lines on the sides of the device, usually outside the active areas of the device. See, e.g., U.S. Pat. No. 4,688,896, assigned to the assignee of the present invention, and which is incorporated herein by reference. In the display device described in that patent and in similar prior art devices, bypassing an open circuit in a defective line requires that one auxiliary line be connected to the defective line at one side of the device and that a second auxiliary line be connected to the defective line on the other side of the device. The auxiliary lines are electrically connected by a jumper or the like, allowing the signal to be applied to the defective line on both sides of the open circuit condition. Such display devices typically can have only a few auxiliary lines compared with the number of data and scan lines in the device. For example, a six inch by six inch imager might commonly have 1,536 data lines and 1,536 scan lines, but only sixteen auxiliary repair lines on a side. Further, repair of one transmission line typically requires use of two auxiliary lines, one on either side of the defective transmission line, and the use of an external jumper to connect the two auxiliary lines together. Once an auxiliary line is used to repair a given line, however, it cannot be used for repair of any other transmission line. Additionally, such a structure cannot be used to repair a transmission line having two or more open circuits along its length.
A second approach to producing repairable liquid crystal display devices has incorporated redundant or backup switching transistors coupled to the active components in the device. For example, the pending U.S. patent application of N. R. Whetten, Ser. No. 07/373,433, filed Jun. 30, 1989, now U.S. Pat. No. 5,062,690 which is assigned to the assignee of the present invention and is incorporated herein by reference, discloses built-in redundant FETs connected by laser-fusible links to the liquid crystal cells in the display device. In such a device, a redundant FET can be connected to a liquid crystal cell if a defect develops which disables the primary FET. Such an approach further provides for use of individually fusible crossover bypass links to be welded together at each crossover point as needed. Other approaches have included use of a complete second set address lines and switching transistors which are electrically connected to opposing sides of the liquid crystal cell at the time of fabrication and thus are in continuous operation, regardless of whether a defect exists on a given address line. Such an arrangement of duplicate lines reduces the amount of space in the display available for the active display devices.
A structure providing a high yield of conductive address lines is disclosed in U.S. Pat. No. 4,804,953, assigned to the assignee of the present invention and incorporated herein by reference. As described in that patent, multilayer repair and/or scan lines can be fabricated having two layers of metal in contact along substantially their entire length. An insulating layer having a width narrower than that of the conducting lines is disposed between them; the upper conducting line overlaps the insulating layer to make contact with the lower conducting line along substantially 90% of its length. Such multilayer lines provide a level of redundancy in the event an open circuit develops in a segment of the upper or lower conducting lines. Such an arrangement, however, does not provide for selective use of the redundant lines as both are in service at all times during operation of the device, and further with such an arrangement it is not possible to isolate a portion of one line to correct a short circuit in the device.
As display devices become larger, with increased numbers of scan and data lines, and as higher resolution is required of the device as a whole, fewer inoperative pixels can be tolerated. Larger devices having multiple switching transistors and associated electrical connections for each liquid crystal cell also involve greater cost and complexity of fabrication. Additionally, in some uses, thin film electronic display devices must be fabricated having connections for drivers on only two sides of the device, i.e., each data and scan line is structured so that it can receive an electrical input signal on only one end. In such devices a conventional auxiliary line arrangement with jumper connections cannot be used to repair a defective transmission line, as the scan and data lines extend outside of the active area of the display on only one side of the device.
It is accordingly an object of this invention to provide a thin film electronic display device structure having repair lines useable in display devices adapted to be driven from only two sides of the array.
It is another object of the present invention to provide a thin film electronic display device structure that does not require redundant or back-up switching transistors to repair a defect in a transmission or address line in a device.
It is another object of the present invention to provide a thin film electronic device structure that readily provides for repair of the device after it has been fabricated.
It is a further object of this invention to provide a novel method for repairing defects in scan or data lines in a thin film display device containing separate repair lines for each structure of the scan and data lines in the device.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read together with the accompanying drawings in which like reference numerals refer to like elements.